In one type of printing method where dampening water is necessary, control of a delicate balance between dampening water and ink is difficult and there are big problems that ink is emulsified or ink is mixed with the dampening water whereby unfavorable concentration of the ink or stain of the ground are generated resulting in loss of paper. On the contrary, a waterless lithographic printing plate precursor does not require dampening water and, therefore, it has many advantages.
On the other hand, as a result of quick progress in an output system in recent years such as prepress system, image setter and laser printer, there have been proposals for the manufacture of printing plate by a new plate-making method such as computer-to-plate and computer-to-cylinder and there has been a demand for printing materials of a new type for such printing systems and development has been carried out.
An example of waterless lithographic printing plate precursors on which writing by laser is possible is a lithographic printing precursor where an ink-repulsive silicone rubber layer is provided on a layer for converting light to heat (hereinafter, referred to as a light-to-heat conversion layer) comprising a layer containing a laser beam-absorber such as carbon black and a binder or a thin metal membrane layer. When laser is irradiated on this lithographic printing plate precursor, a silicone rubber layer which is the irradiated part is removed to give an ink-attached region (image area) while non-irradiated part which is a region where silicone rubber layer remains becomes an ink-repulsive region (non-image area) whereupon a printing without water is now possible.
In such a waterless lithographic printing plate precursor, cost of the manufacture is low and, in addition, image is formed by utilizing the abrasion of light-to-heat conversion layer of the laser-irradiated part and, therefore, there is an advantage that generated gas pushes up the silicone rubber layer of the laser irradiated part and removal of the silicone rubber layer of the laser irradiated part thereafter (hereinafter, referred to as development) is able to be carried out efficiently.
There is also a disclosure for an embodiment that such a waterless lithographic printing plate precursor is made into a roll shape and attached in the plate drum of a printer, supplied on the plate drum so as to make the printing surface of the waterless lithographic printing plate precursor front and spooled so as to position the new side of the waterless lithographic printing plate precursor to a printing region of a plate drum, laser is scanned in a image-forming manner on the aforementioned plate drum and printing is conducted after the silicone rubber layer of the laser irradiated part is removed (for example, refer to WO 90/02045).
However, in such a waterless lithographic printing plate precursor, troubles of scratching of non-image area during a developing treatment step after writing and recording by laser and a printing step are apt to happen. For example, when the developing treatment step is in such a system that a plate surface is scrubbed by a brush or a pad for development impregnated with a developing treatment solution whereby the silicone rubber layer at the laser irradiated area is removed, fine cracks and exfoliations are generated on the non-image area by dust attached to the pad or the like for development resulting in a ink stain of the print.
As a means for improving the aforementioned scratching of silicone rubber layer, application of corona discharge treatment to a support has been proposed and there is a description that the condition for the corona discharge treatment (treating amount) of 1 W/m2/minute to 200 W/m2/minute is effective (refer to JP 11/245,529 A). It has been also known that, in an lithographic printing plate precursor having a heat sensitive layer containing carbon black and polymer binder on a support and having a hydrophilic layer or a lipophobic layer on the heat sensitive layer, resistance to scratch, resistance to printing and image reproduction property are good when a substance where carbon black is oxidized is used (refer to JP 2002/240,452 A).
However, in the aforementioned corona discharge, useless damage and residue are resulted on the surface of the support so that close adhesion of the support to the light-to-heat conversion layer is deteriorated whereby there are some cases where non-image area is scratched and staining with printing ink is generated. In addition, improvement in resistance to scratch by carbon black which is subjected to an oxidizing treatment gives still insufficient effect.